Electrical discharge treatments are widely used to promote adhesion of a variety of organic and inorganic layers to organic polymer substrates. Examples of the use of electrical discharge treatments are found in U.S. Pat. No. 5,538,841 and references cited therein. Additional examples are found in European Pat. Application EP 0 758 687 A1 and references cited therein, as well as well as World Pat. WO 97/42257. A variety of treatment geometries (i.e. positioning of the article to be treated relative to the discharge electrodes, shape of the electrodes, and shape of the article to be treated) are possible (see, for example U.S. Pat. Nos. 3,288,638 and 3,309,299) the need to treat continuous sheets or rolls of polymeric support material (i.e., webs) has generally led to treatment apparatus design for the purposes of conveying a web through an electrical discharge zone. This purpose has been achieved either by suspending the polymer article in a free span between conveyance rollers, as disclosed in U.S. Pat. No. 5,493,117 or on a drum, as disclosed in U.S. Pat. No. 4,451,497 and U.S. Pat. No. 5,224,441. In U.S. Pat. Nos. 4,451,497 and 5,493,117, as well as in U.S. Pat. No. 5,538,841, an objective of the invention is to provide surface treatments for use in the manufacture of photographic imaging elements on polyester supports. Dolazalek et al (U.S. Pat. No. 4,451,497) disclose an apparatus for conveying a polymer web material into a vacuum chamber, through a treatment zone and out of the vacuum chamber. The treatment configuration shown and discussed is essentially a corona treatment geometry wherein the web travels along a rotating drum that is surrounded by a plurality of discharge electrodes. The objective is to prepare a substrate to be coated with photographic emulsion. Tamaki et al. (U.S. Pat. No. 5,493,117) disclose an apparatus similar to that of Dolazalek et al. having the similar purpose of providing a support useable for a photosensitive material. In their treatment zone, however, Tamaki et al. suspend the web in free span between conveyance rollers and have a plurality of treatment electrodes located on either side of the free span in order to treat both sides of the web simultaneously. Felts et al (U.S. Pat. No. 5,224,441) disclose a plasma treatment and coating apparatus wherein the web is conveyed over the surface of an electrified drum, facing a grounded counter electrode. Grace et al. (U.S. Pat. No. 5,538,841) disclose nitrogen-based and oxygen-based surface chemistries that promote adhesion of gelatin-containing layers to respective nitrogen-plasma-treated and oxygen-plasma-treated polyester webs, also for the manufacture of supports usable for photosensitive materials.
The method of conveyance of the web material through the treatment zone has an important effect on the nature of the plasma treatment. In the case of Tamaki et al., the polymer surface to be treated is electrically floating in the discharge zone and moves past one or more powered discharge electrodes. In the case of Dolazalek et al., if the drum is electrically isolated from the walls of the apparatus, the article also is electrically floating in the discharge zone and moves past one or more powered discharge electrodes. If the drum is electrically grounded, however, the surface potential of the polymer article is determined by several factors. These factors include thickness and dielectric properties of the article, the driving frequency of the discharge, the electron density and plasma potential of the discharge, and the relative areas of the discharge electrode and the combination of the drum surface and the grounded inner walls of the apparatus. At sufficiently low driving frequency (the upper limit being determined by the aforementioned characteristics of the article and plasma), the article surface will charge to the floating potential and the situation will be similar to that of an electrically isolated drum. At sufficiently high driving frequency (the lower limit being determined by aforementioned characteristics of the article and plasma) the surface of the article will remain near ground potential. Consequently, if the effective grounded surface area in the discharge zone is significantly larger than that of the powered electrode(s), the surface of the article to be treated is generally bombarded by ions having a bombardment energy that is largely determined by the difference between a plasma potential of some tens of volts and a ground potential.
In contrast, if the areas of the powered electrode(s) and the effective grounded electrode are comparable, the ion bombardment of the polymer article will be largely determined by the potential applied to the powered electrode and can have a peak value of several hundred volts or more. In this case, the ion bombardment energies are more characteristic of an etch process. The etching character of the process can be further enhanced by reducing the area of the polymer article, supporting electrode (e.g., drum), and effective grounded surface area relative to that of the driven electrode(s), or by electrically isolating the supporting electrode of reduced area and applying the driving voltage thereto. The effect of the relative areas of driven and grounded electrodes on the effective bombarding potentials at their respective surfaces is well known to those skilled in the art of plasma processing for microelectronics. In that art it is known that alternating-current discharges established between a driven electrode and a ground electrode of equal size produce similar bombardment effects at either electrode. It is also known that alternating-current discharges established between electrodes of dissimilar area produce more bombardment at the smaller electrode. It is further known that higher frequency discharges operate at lower amplitudes of driving voltage (for comparable input power). Thus the bombarding potential is also reduced as driving frequency increases. The aforementioned behavior of the bombarding potential at the electrode surfaces applies reasonably well throughout the radio frequency range (i.e., .about.3 kHz to .about.100 MHz). For the purposes of surface modification of polymer webs, treatments in which significant ion bombardment and etch processes may occur have generally been avoided in the prior art related to supports for photographic elements. While the object of polymer surface modification is generally to introduce new chemical species into the surface region by reaction with species in the electrical discharge, the object of etch processes is to remove significant amounts of material from the surface region. Furthermore, these etch processes are considered undesirable because the materials to be modified tend to be temperature sensitive and etch processes can generate substantial heat or may generate considerable low-molecular-weight fragments in the surface region of the treated support.
Examples of background art that teach away from using etch-like processes for the purposes of polymer surface modification are found in the open literature. J. E. Klemberg-Sapieha et al (J. Vac. Sci. Technol. A, 9 (6) (1991) 2975-81) disclose a dual-frequency approach to modification of polymer surfaces in nitrogen plasma and in ammonia plasmas. In their work, the high-frequency microwave power couples effectively to the bulk of the discharge zone and generates the chemically active species in the plasma. By applying a lower frequency (rf: 13.56 MHz) potential to the substrate holder, they create a significant bias potential, which results in significant bombardment of the substrate (i.e. polymer article being treated) by ions extracted from the plasma. Their work shows clearly that the maximum amount of nitrogen incorporated into the treated polymer surface is for the microwave plasma with no applied rf potential. As the rf potential is applied (resulting in bias potentials of up to 500 V) the incorporated nitrogen decreases for both nitrogen and ammonia plasmas and for both polymers studied in their work (i.e., polyethylene and polyimide). While dual-frequency approach has shown much success for tailoring the properties of hard coatings such as silicon nitride and diamond-like carbon (see for example, J. E. Klemberg-Sapieha et al. in Rarefied Gas Dynamics: Experimental Techniques and Physical Systems, B. D. Shizgal and D. P. Weaver, eds., Progress in Astronautics and Aeronautics, vol. 158, A. R. Seebass, Editor-in-chief, American Institute of Aeronautics and Astronautics, Inc., 1993), the application of this approach to polymer surface modification has suggested that enhanced ion bombardment by use of a low-frequency bias is generally disadvantageous.
Another example of using a plasma source with a separately biased sample holder is found in the work of S. Han et al. (Surface Coatings Technology 93 (1997) 261-4) and Lee et al. (J. Vac. Sci. Technol. A 16(3), (1998) 1710-15). Han et al and Lee et al use an rf (13.56 MHz) inductively coupled plasma source (with magnetic enhancement) in combination with a pulse generator used to apply short (10-20 .mu.s) high-voltage (up to -10 kV) pulses to the substrate holder. In their work, Han et al. and Lee et al. found that the use of high-voltage pulses alone to generate a discharge and effect surface modification is less effective than the use of the rf plasma in combination with high-voltage pulses applied to the substrate holder. Furthermore, they found that the use of the rf-driven plasma alone is less effective than in combination with the high-voltage pulses.
The results for the use of the high-voltage pulses alone are consistent with the findings of Klemberg-Sapieha et al. (described above) that applying a bias voltage to the substrate holder is not advantageous for polymer surface modification. In contrast, the results for the combination of an rf plasma and high-voltage pulses as compared to the rf plasma alone appears to show some interesting effects. The apparatus as described by Han et al. and Lee et al., however, has several drawbacks. First, the high-voltage pulses are short (microseconds) and must be applied repetitively (1 kHz, e.g.) for significant time (several minutes) to modify polymer surfaces to the degree shown by Han. et al and Lee et al. Second, the apparatus requires rf power to be applied in an inductively coupled configuration, high-voltage pulsing electronics, and permanent magnets. Third, the apparatus as described is clearly designed to treat small articles such as silicon wafers, as opposed to wide continuous rolls of web. All of the above drawbacks present complications for application of this technology to high-speed treatment of polymer supports.
Grace et al. disclose the use of nitrogen plasmas and oxygen plasmas to treat polyester supports for promoting the adhesion of aqueous coatings thereto. Using low-frequency (60 Hz-40 kHz) discharges with the polyester support electrically floating in the plasma, Grace et al. found nitrogen-containing and oxygen-containing surfaces that are demonstrated to promote excellent adhesion between gelatin-containing layers and plasma-treated polyester supports. The doses demonstrated to produce good adhesion are in the range of 0.5-4 J/cm.sup.2. (This dose parameter is calculated based on the delivered power, the width of the treatment zone and the web speed: Dose=Power /[width.times.web speed].) In this dose range, treatment times of 1 s and somewhat below are readily attainable on the manufacturing scale. Grace et al. teach the appropriate surface chemistry for the given application and disclose optimized treatment parameters. Neither the use of substrate bias voltage, nor the use of an etch-mode plasma treatment are disclosed, nor is it suggested that such approaches should be more effective at obtaining the desired surface chemistry. The present invention relates to the efficient production of surfaces bearing chemical similarity to those disclosed by Grace et al.